Re: [PATCH v2 2/6] Documentation: devicetree: add bindings to support ARM MHU doorbells

[Jassi Brar]

On Wed, May 24, 2017 at 3:46 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> The ARM MHU has mechanism to assert interrupt signals to facilitate
> inter-processor message based communication. It drives the signal using
> a 32-bit register, with all 32-bits logically ORed together. It also
> enables software to set, clear and check the status of each of the bits
> of this register independently. Each bit of the register can be
> associated with a type of event that can contribute to raising the
> interrupt thereby allowing it to be used as independent doorbells.
>
> Since the first version of this binding can't support doorbells,
> this patch extends the existing binding to support them.
>
> Cc: Alexey Klimov <alexey.klimov@xxxxxxx>
> Cc: Jassi Brar <jaswinder.singh@xxxxxxxxxx>
> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> Cc: devicetree@xxxxxxxxxxxxxxx
> Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> ---
>  .../devicetree/bindings/mailbox/arm-mhu.txt        | 46 ++++++++++++++++++++--
>  1 file changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/mailbox/arm-mhu.txt b/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
> index 4971f03f0b33..bd9a3a267caf 100644
> --- a/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
> +++ b/Documentation/devicetree/bindings/mailbox/arm-mhu.txt
> @@ -10,21 +10,42 @@ STAT register and the remote clears it after having read the data.
>  The last channel is specified to be a 'Secure' resource, hence can't be
>  used by Linux running NS.
>
> +The MHU drives the interrupt signal using a 32-bit register, with all
> +32-bits logically ORed together. It provides a set of registers to
> +enable software to set, clear and check the status of each of the bits
> +of this register independently. The use of 32 bits per interrupt line
> +enables software to provide more information about the source of the
> +interrupt. For example, each bit of the register can be associated with
> +a type of event that can contribute to raising the interrupt. Each of
> +the 32-bits can be used as "doorbell" to alert the remote processor.
> +
>  Mailbox Device Node:
>  ====================
>
>  Required properties:
>  --------------------
> -- compatible:          Shall be "arm,mhu" & "arm,primecell"
> +- compatible:          Shall be "arm,primecell" and one of the below:
> +                       "arm,mhu" - if the controller doesn't support
> +                                   doorbell model
> +                       "arm,mhu-doorbell" - if the controller supports
> +                                   doorbell model
>  - reg:                 Contains the mailbox register address range (base
>                         address and length)
> -- #mbox-cells          Shall be 1 - the index of the channel needed.
> +- #mbox-cells          Shall be 1 - the index of the channel needed when
> +                       compatible is "arm,mhu"
> +                       Shall be 2 - the index of the channel needed, and
> +                       the index of the doorbell bit with the channel when
> +                       compatible is "arm,mhu-doorbell"
>  - interrupts:          Contains the interrupt information corresponding to
> -                       each of the 3 links of MHU.
> +                       each of the 3 physical channels of MHU namely low
> +                       priority non-secure, high priority non-secure and
> +                       secure channels.
>
>  Example:
>  --------
>
> +1. Controller which doesn't support doorbells
> +
>         mhu: mailbox@2b1f0000 {
>                 #mbox-cells = <1>;
>                 compatible = "arm,mhu", "arm,primecell";
> @@ -41,3 +62,22 @@ used by Linux running NS.
>                 reg = <0 0x2e000000 0x4000>;
>                 mboxes = <&mhu 1>; /* HP-NonSecure */
>         };
> +
> +2. Controller which supports doorbells
> +
> +       mhu: mailbox@2b1f0000 {
> +               #mbox-cells = <2>;
> +               compatible = "arm,mhu-doorbell", "arm,primecell";
> +               reg = <0 0x2b1f0000 0x1000>;
> +               interrupts = <0 36 4>, /* LP-NonSecure */
> +                            <0 35 4>, /* HP-NonSecure */
> +                            <0 37 4>; /* Secure */
> +               clocks = <&clock 0 2 1>;
> +               clock-names = "apb_pclk";
> +       };
> +
> +       mhu_client: scb@2e000000 {
> +               compatible = "arm,scpi";
> +               reg = <0 0x2e000000 0x200>;
> +               mboxes = <&mhu 1 4>; /* HP-NonSecure 5th doorbell bit */
> +       };
>
Every MHU controller can by driven as "arm,mhu-doorbell" or "arm,mhu"
equally fine. So you are basically smuggling a s/w feature into DT.